To meet requirements for compact electronic instruments, high density mounting on a printed board, efficient mounting and the like, manufacture of electronic parts in a chip form and in a small size is drastically proceeding and accompanying this, demands for manufacture of electrolytic capacitors used as the parts in a chip form and in a reduced size are increasing. For this purpose and also in view of easy handleability, solid electrolytic capacitors not using an electrolytic solution are abruptly growing in recent years.
In the fabrication of a solid electrolytic capacitor in general, the surface of anode material comprising a valve-acting metal such as aluminum, tantalum, niobium or titanium is etched to roughen the surface, thereby forming pits in the micron order and enlarging the surface area, an oxide dielectric film is formed thereon by an electrochemical forming, a solid electrolytic solution working out to a cathode material is impregnated between the oxide dielectric film and the anode moiety through a separator (masking), the obtained capacitor element is coiled or stacked and then housed in a cylindrical metal case, and the opening of the metal case is sealed with a sealing member. In the case of a chip-type solid electrolytic capacitor, a solid electrolyte is impregnated into an electrolytic foil having formed thereon an oxide dielectric film, a cathode electrically conducting layer comprising a carbon paste layer and silver paste is formed thereon, and then the outer jacket is formed.
Among the above-described valve-acting metals, aluminum has the advantages that the surface area can be easily enlarged by etching, the oxide film formed on the surface by anodization (electrochemical forming) using the aluminum as anode can be utilized as a dielectric material and therefore, a small-size and large-capacitance solid electrolytic capacitor can be inexpensively produced as compared with other capacitors. From these reasons, aluminum solid electrolytic capacitors for low voltage are being widely used.
The electrode foil used in the aluminum solid electrolytic capacitor is obtained by electrochemically or chemically etching an aluminum foil to enlarge the surface area, stamping it out into a product pattern shape and electrochemically forming the cut end portion of the foil.
The method for etching the aluminum foil includes a D.C. (direct current) electrolytic etching method of performing the etching by passing a direct current through an electrolytic solution comprising a chloride ion-containing aqueous solution having added thereto phosphorus acid, sulfuric acid, nitric acid or the like, using the aluminum foil as a positive electrode and an electrode disposed adjacent to the aluminum foil as a negative electrode; and an A.C. (alternating current) electrolytic etching method of performing the etching by applying an A.C. voltage between electrodes disposed at both ends of the aluminum foil in an electrolytic solution comprising a chloride ion-containing aqueous solution having added thereto phosphorus acid, sulfuric acid, nitric acid or the like.
In the D.C. electrolytic etching, corrosion proceeds like a tunnel in the direction perpendicular to the aluminum surface. On other hand, in the A.C. electrolytic etching, corrosion proceeds like a rosary running in the random direction and this is advantageous for the enlargement of the surface area (surface enlargement). Therefore, A.C. electrolytic etching on aluminum foil is predominating. However, a method of combining these two methods and a method of gradually elevating the A.C. voltage are also proposed (see, JP-A-11-307400 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). In addition, a method of enhancing the effective surface enlargement by designing the waveform, amplitude and the like of the alternating current (see, JP-A-7-235456 (U.S. Pat. No. 5,500,101)) and a method of using aluminum containing a specific metal serving as a starting point of corrosion are proposed (see, JP-A-7-169657).
The present inventors have previously proposed a production method of a solid electrolytic capacitor element, where one side of each metal foil (aluminum foil) cut into a predetermined rectangular shape are fixed to a metal-made linear support (metal-made guide) to lay the metal foils in a row, the metal foil in the region where a solid electrolyte is formed is entirely subjected to an electrochemical forming, a masking treatment is applied to the boundary between the electrochemically formed region where a solid electrolyte is formed and the region which works out to an anode terminal, and then a solid electrolyte is formed. According to this method, the electrochemical forming even in the cut end portion generated by the cutting can be completely attained and therefore, a leakage current due to invasion of solid electrolyte or electrically conducting paste into the cut end portion can be prevented.
In the manufacture of a solid electrolytic capacitor by the above-described method, a plurality of elements with the cut end portion being electrochemically formed are generally stacked for obtaining a capacitor having a predetermined capacitance, an anode leading wire is connected to the anode terminal, a cathode leading wire is connected to the electrically conducting layer containing an electrically conducting polymer, and the device as a whole is molded with an insulating resin such as epoxy resin to manufacture a solid electrolytic capacitor.
In this solid electrolytic capacitor, the cut end portion in the non-etched state is electrochemically formed, so that unless the polymerization conditions are precisely controlled during the bonding process of an electrically conducting polymer in the cathode moiety, the electrically conducting polymer is not uniformly formed or is liable to extend over the masking and adhere to the anode moiety and this gives rise to problems such as decrease in the capacitance and increase in the equivalent series resistance (ESR) and in the leakage current. Furthermore, the sagging of cut end part at the cutting incurs worsening of the above-described capacitor characteristics.
In the case where the anode moiety of a solid electrolytic capacitor element is etched and electrochemically formed to form a dielectric layer, a connection failure may arise in connecting the anode moieties of a plurality of elements.
Accordingly, an object of the present invention is to solve the above-described problems of conventional techniques and provide a method for producing an aluminum foil for solid electrolytic capacitors, which can increase the capacitance of a solid electrolytic capacitor, ensure stable quality and elevate the productivity.
Under the circumstances, the present inventors have made extensive research on a method for producing an aluminum foil for solid electrolytic capacitors from an aluminum sheet having a non-etched surface and an aluminum sheet having an etched surface, both of which are commercially available as material sheets as aluminum foil for solid electrolytic capacitors.
As a result, the present inventors have found that in both the aluminum sheet having a non-etched surface and aluminum sheet having an etched surface, it is effective to etch a cut end portion of the aluminum foil previously cut into a form of capacitor element.
Also, they have found that in the case of the aluminum sheet having a non-etched surface, when the cut end portion formed by the cutting and the area used for the formation of an electrically conducting layer exclusive of the anode terminal on an aluminum foil previously cut into a predetermined rectangular shape is dipped in an electrolytic solution to perform the electrolytic etching and then electrochemically formed at a predetermined voltage, the entire surface can be electrochemically formed uniformly to afford increase in the capacitor capacitance and decrease in the leakage current, the anode moieties can be efficiently connected without fail on stacking elements and the productivity can be elevated.
Furthermore, they have found that in the case of the aluminum sheet having an etched surface, when only the cut end part is etched and then electrochemically formed at a predetermined voltage, the entire surface can be electrochemically formed uniformly to afford an increase in the capacitor capacitance and a decrease in the leakage current.